net: convert standalone IsSelectableSocket() to Sock::IsSelectable()

This makes the callers mockable.

src/netbase.cpp

@@ -503,7 +503,7 @@ std::unique_ptr<Sock> CreateSockTCP(const CService& address_family)
 
     // Ensure that waiting for I/O on this socket won't result in undefined
     // behavior.
-    if (!IsSelectableSocket(sock->Get())) {
+    if (!sock->IsSelectable()) {
         LogPrintf("Cannot create connection: non-selectable socket created (fd >= FD_SETSIZE ?)\n");
         return nullptr;
     }